ISSI Team

Massive stars are among the most important cosmic engines: they trigger the star formation and, together with low-mass stars, enrich the interstellar medium with the heavy elements but on short time scales, ultimately leading to formation of Earth-like planets and development of life.

Among the bright X-ray sources in the sky a significant number consists of a compact object accreting from the wind of such massive stars. These winds are fast, with typical terminal velocities up to 2500 km/s , dense, with mass-loss rates M >10⁻⁷M_⊙/yr , and driven by line scattering of the star’s intense continuum radiation field. Good examples are Cyg X-1/HDE 226868, the first detected stellar-mass black hole, and Vela X-1, one of the most important accreting neutron star binaries. While the basic picture has been established for decades, many details are still debated.

The birth, life, and death of massive stars (M > 10M_⊙) are deeply interwoven with the evolution of star clusters and galaxies. Massive stars generate most of the ultraviolet radiation of galaxies — the whole Universe was reionized with the help of the first (super)massive stars — and power their infrared luminosities. Massive star winds and final explosions as supernovae provide a significant input of mechanical and radiative energy into the interstellar medium, and play a crucial role in the evolution of star clusters and galaxies. Thus, massive stars are among the most important cosmic engines: they trigger the star formation and, together with low-mass stars, enrich the interstellar medium with the heavy elements but on short time scales, ultimately leading to formation of Earth-like planets and development of life.

Among the bright X-ray sources in the sky a significant number consists of a compact object accreting from the wind of such massive stars. These winds are fast, with typical terminal velocities up to 2500 km/s, dense, with mass-loss rates ≈ 10⁻⁷ M_⊙/yr, and driven by line scattering of the intense continuum radiation field of the star. Good examples are Cyg X-1/HDE 226868, the first detected stellar-mass black hole, and Vela X-1, one of the most important accreting neutron star binaries. While the basic picture has been established for decades, many details are still debated.

In 2013 and 2014 the ISSI team ”Unified View of Stellar Winds in Massive X-ray Binaries” brought together experts in the field of massive star winds and accreting high mass X-ray binaries. The main goal was to bridge the gap between these two communities in order to consolidate the understanding of fast outflows from massive stars. Thanks to the opportunity offered by ISSI, a solid framework of observational and theoretical results was compiled and research activities along these lines were boosted. But at the same time, the meetings and discussions unveiled major discrepancies between results obtained in high-mass X-ray binaries and in isolated massive stars, which cannot be explained by differences in spectral types. Thus, important physical parameters controlling the wind structure are still not sufficiently understood. This proposal aims at promoting further tight collaborations within a reshaped ISSI team, including new team members with expertise in fields that have been found to be missing in the previous team, as well as debating about new observations.

Based on the findings of the first completed program, the team has been reorganized to cover identified gaps in the expertise, namely hydrodynamic modeling of wind accretion in HMXBs, accretion theory and evolution of massive stars. Moreover, new data are available and in particular simultaneous X-ray and optical spectroscopic data, that will allow us to probe the feedback mechanisms between the X-ray pulsar and the wind structures in sub-minute time-scales for the first time. With the new team and the new data, we propose to tackle the following goals:

• Diminish uncertainties of clump properties and minimize the impact on the estimates of mass loss rates from massive stars.
  Verify in how far the size of clumps and other macro-structures in the winds (e.g. CIRs) can be enhanced and/or disrupted by the presence of the neutron star. Combining the expertise of team members Manousakis, Puls and Sundqvist, a multi-dimensional hydrodynamic simulation of a clumped wind interacting with an X-ray producing compact object will be attempted. Observers in the team will participate by providing key observational properties that such simulations should be able to reproduce.
• Solve the drastic discrepancy in inferred wind velocities in single stars and HMXBs.
  Ionization of the wind material by X-rays from the neutron star was identified as the most probable cause for this discrepancy during the first ISSI program. A code in development (Bozzo, Oskinova) will allow to estimate the degree of ionization in the wind as a function of the orbital properties of the binary system and neutron star parameters (mass, radius, magnetic field intensity, spin period, and X-ray luminosity). Observers will help testing different code parameters against observational findings.
• Update empirical tools to study stellar winds by incorporating the findings of the above points.
  Results about the presence of dense clumps, structures, faster/slower outflows in the wind of massive stars shall provide guidance for the production of new synthetic spectra to be compared with observations of close-by massive stars (for which high-quality spectroscopic data are available).
• Compare different models of wind-accretion (Bondi-Hoyle vs Quasi-Spherical) with experts present.
• A comprehensive empirical picture of the wind of massive stars will provide direct constraints for population synthesis studies.